You can't call them 아저씨, 아줌마!!
Call them 삼촌, 이모
Uncle = 삼촌 aunt = 이모, 고모, 숙모
In Korea, there are 3 way to call aunt.
이모 = When you call mother's sister
숙모 = When you call uncle's wife
고모 = When you call father's sister
But I think you don't have to memorize these.
Just keep '이모'.